There is growing interest in optical communication systems utilising the 850 nm communications window provided by silica optical fibres.
One known approach to providing optical sources or amplifiers is to use a semiconductor laser as the source of a pump signal for an optical fibre laser or amplifier which then emits or amplifies an optical signals at a wavelength which may be the same as or different to that of the pump. This has the advantage of providing ready access to the signal wavelength in a manner which can take advantage of the relative low cost, high efficiency and commercial availability of laser diodes.
Lasers and amplifiers are known which depend for their operation on lasing transitions between upper and lower lasing levels of ions in a lasing medium each of which lies above the levels of the ions' ground state. If the average lifetime of ions in the upper lasing level (ULL) is longer than the lifetime of ions in the lower lasing level (LLL) then pumping of the laser medium to populate the ULL by excitation of ions from their ground state, perhaps via a level above the ULL, can be employed to maintain a population inversion between the ULL and LLL to allow lasing action to proceed. If, however, the lifetime of the ULL is shorter than the LLL, the lasing transition is known as self-terminating because the build up of ions in the LLL destroys the population inversion between the ULL and LLL required for lasing. Consequently such self-terminating lasers normally only operate in pulsed form, the LLL being given time to empty between pumping pulses.
The applicant's published application WO 89/11744 discloses a fluoro-zirconate optical fibre laser and amplifier in which cw operation of the normally self-terminating transition .sup.4 I.sub.11/2 to .sup.4 I.sub.13/2 is achieved by applying excitation energy to elevate ions from the lower lasing level to maintain a population inversion between the upper and lower lasing levels during lasing.
This use of excited state absorption (ESA) to provide the means of maintaining a population inversion to produce cw lasing eliminates the need to use high dopant concentrations necessary to obtain up-conversion from the LLL by ion-ion energy transfer. Instead, low dopant concentrations can be employed pumped highly efficiently by a laser for example. This is particularly the case if the laser medium comprises a doped optical fibre as the waveguiding properties of the fibre means high power densities can be maintained over large interaction lengths. This is also thermally efficient as the fibre core has a high surface-to-volume ratio.
This arrangement requires the pump means also to provide pump energy at a wavelength and with an intensity sufficient to excite ions from the ground state to the ULL, perhaps via a more energetic level.
With the energy levels of some ions in a particular host, as discussed in the above referenced application, the wavelength and intensity of the pump excitation energy which elevates ions from the LLL can be chosen so that it also coincides with the energy difference between the ground state and an energy level, perhaps the ULL itself, which results in populating the ULL. This provides a much simplified arrangement in that a single wavelength source can be used to pump the ions to the ULL and to depopulate the LLL to prevent saturation. This coincidence of energy levels is not always present, however.